Method for object classification using polarimetric radar data and device suitable therefor

ABSTRACT

The invention relates to a method for object classification which comprises the following steps for providing an ellipti-cally or circularly polarized transmission signal which is transmitted to the object to be classified: generating a first radar image from the copolarly polarized reflection signal and generating a second radar image from the cross-polarized reflection signal and comparing the first radar image with the second radar image.

The present invention relates to a method for object classification with polarimetric radar data and to a device suitable therefor.

It is generally known to use radars with linearly polarized signals for object classification. The results achievable in this case are, for example, in the case of recorded radar images not unique, or, in respect of different objects, ambiguous.

It is therefore an object of the present invention to provide a method for object classification, which reduces the disadvantages existing in the prior art, and a suitable device therefor. It is furthermore an object of the present invention to increase the correlation in the object classification and furthermore optionally to provide data which may be used for a further versatile application possibility.

The aforementioned objects are achieved in relation to the method by the features of claim 1 and in relation to the device by the features of claim 12.

In this case, it has been found that by providing an elliptically or circularly polarized transmission signal, which is transmitted onto the object to be classified, correspondingly different reflection signals are employed in order to generate different radar images, which may then be compared. The effect achieved by this measure is that prominent object regions can be distinguished and an improved object classification can therefore be brought about.

The method according to the application and the device according to the application may therefore be employed in future radar sensors, which may be used particularly in highly automated and autonomous driving.

To this end, polarimetric radar sensors are required, which are distinguished particularly in that, significantly more target information can be generated with them in comparison with currently used radars with linearly polarized signals. This is based on the fact that mutually independent radar images can be generated for the co- and cross-polarization, and there is a higher target detection probability with circular polarization.

The method according to the invention involves evaluating polarimetric radar data in respect of pattern recognition for classifying different objects, as well as the detection of so-called “ghost targets”. The latter are caused by multipath propagations, side lobes and by periodically recurring main lobes (so-called grating lobes).

The physical principle according to the invention is shown by FIG. 1. A circular or elliptically polarized wave is transmitted, and a primarily cross-polarized wave or a primarily copolarized wave is received, depending on the structure of the target. In the case of an odd number of reflections at the target, the polarization direction is rotated, and in the case of an even number of reflections the same polarization is received back. If a left-circular wave is transmitted, for example, the cross-polarization is a right-circular wave and the copolarization is a left-circular wave. A description of this principle and of the structure of a polarimetric radar sensor may be found in [1].

So that this principle can be implemented, a transmitter is required which transmits at least one left-circular or right-circular or elliptical polarization. The receiver must in this case be fully polarimetric. This means that circular, elliptical and linear polarizations can be received. This may be achieved by the reception of left-circular and right-circular components of the reception signal. All polarizations may then be represented by means of the ratio of the left- and right-circular components. A further possibility for receiving fully polarimetrically is the reception of vertically and horizontally linearly polarized components of the reception signal. So that all polarizations can be represented, in this case the magnitude and the phase of the vertically linearly and horizontally linearly polarized components of the reception signal must be evaluated.

FIG. 2 shows the result of a circularly polarimetric measurement with reference to the example of an automobile. Two independent radar images are obtained, a copolar radar image and a cross-polar radar image. In the copolar radar image, even reflections on the target are represented, primarily double reflections. In the cross-polar radar image, odd reflections on the target are represented, primarily single reflections. In this case, in both cases local maxima of the amplitudes of the signal reflected back are represented by circles, the diameter of which is in proportion to their amplitude.

FIG. 3a and FIG. 3b show a polarimetric radar image which contains both the copolar and the cross-polar local maxima of FIG. 2. Now, however, the magnitude ratio between co- and cross-polarized signal components (referred to below as the “ratio”) is additionally represented in the form of various symbols for each local maxima. A polarimetric pattern, with which objects can be classified, is thereby formed at different regions of the automobile.

During pattern recognition for various regions of the targets, the following properties of the local maxima are evaluated:

-   -   number of copolar maxima, in this case determined         signal-to-noise ratios (distance from the noise level) may also         be incorporated     -   number of cross-polar maxima, in this case determined         signal-to-noise ratios (distance from the noise level) may also         be incorporated     -   average magnitude ratio between co- and cross-polarization     -   maximum magnitude ratio between co- and cross-polarization     -   minimum magnitude ratio between co- and cross-polarization     -   phase ratios between co- and cross-polarization     -   particular characteristic properties.

The latter is, for example, the first reflection on the automobile in the region of the front license plate. This is a ratio of less than −20 dB.

The pattern classification in this case distinguishes different object types, for example automobiles, pedestrians, cyclists, trucks and motorcyclists and road engineering targets, for example drains, barriers, guardrails, bridges and tunnels.

FIG. 4 shows the automobile measured frontally and with an angle offset of −20°. For the angle offset of −20°, the following change is to be found relative to the frontally measured automobile:

-   -   the characteristic reflection on the front with the ratio less         than −20 dB is shifted to the right-hand side of the automobile     -   reflections occur in the region of the front with ratio values         of between 10 dB and 15 dB, as well as a reflection with a ratio         of between 15 dB and 20 dB. (This is due to the strong copolar         properties. These occur because there are more double         reflections in the region of the front, particularly in the         region of the car grille, because of the obliquely placed         vehicle)     -   there is a change in the polarimetric pattern above all in the         region of the front, of the steering wheel and in the rear         region of the vehicle.

With the aid of these properties, it is possible to establish the angle which the vehicle is at in relation to the sensor.

FIG. 5 shows regions which are of particular importance in the pattern recognition and the classification of a frontally measured automobile. These are:

-   -   front region     -   front wheel well     -   steering-wheel region     -   side mirror.

Characteristic in this case, corresponding to FIG. 3, is a strong reflection in the region of the front license plate with a very low ratio (strong single reflection).

FIG. 6 shows regions which are of particular importance in the pattern recognition and the classification of an obliquely detected automobile:

-   -   front region     -   the wheel wells facing toward the sensor     -   the front door crack facing toward the sensor     -   the rear vehicle corner facing toward the sensor.

Particularly characteristic in this case is the detection of the contour of the vehicle as an L-shape, the exact position detection of the wheel wells, as well as the occurrence of relatively many (in comparison with other measurement positions) signals with a high ratio (double reflections).

FIG. 7 shows regions which are of particular importance in the pattern recognition and the classification of a transversely detected automobile:

-   -   the transverse region facing toward the sensor     -   the wheel wells facing toward the sensor     -   the vehicle corners facing toward the sensor     -   the front door facing toward the sensor.

Particularly characteristic in this case is the detection of strong reflections with a very low ratio (strong single reflections) in the region of the front door.

FIG. 8 shows regions which are of particular importance in the pattern recognition and the classification of an automobile detected from behind:

-   -   rear vehicle region     -   the vehicle corners facing toward the sensor     -   the front door facing toward the sensor.

Particularly characteristic in this case is the detection of a strong reflection with a very low ratio (strong single reflections) on the outer contour of the tale, as well as a polarimetric pattern which comes from the interior of the automobile.

Typical polarimetric patterns with the described properties of object classes, or object subclasses, are in this case always assigned to different angle and distance ranges and are used as a basis for a classification algorithm.

Advantages are furthermore obtained when using circular polarization for detection by “ghost targets” which result from multipath propagation or side lobes or disturbing recurring main lobes. The latter two are particularly strongly pronounced when strong targets are detected with large angle offsets.

FIG. 9 shows by way of example the situation with multipath propagation due to an additional reflection on an object. This leads to a reflected target which is at a different angle. However, the additional reflection also causes a rotation of the polarization properties, so that analysis of the modified polarization pattern allows identification of multipath propagation.

FIG. 10 shows the same situation as in FIG. 9, but relates exclusively to targets with a relative speed with respect to the radar sensor and the environment. If, for targets within a distance and angle gate, a local maximum has different speeds in the copolar signal and in the cross-polar signal, the target is a “ghost target”. These may be caused both by multipath propagation and by side lobes or recurring main lobes.

In the case of radar sensors which transmit circularly polarimetric or elliptical waves, the signals reflected back may be decomposed into a left- and a right-rotating component. A polarimetric pattern is thereby obtained, which may be used for object classification. So that the left-rotating and right-rotating components can be received, one obvious implementation is to provide corresponding reception channels for the two polarizations. This, however, leads to a significant disadvantage. Compared with a linear radar system, two times the number of reception channels are required for the same angle resolution.

A solution approach without this disadvantage is provided by the method according to the invention. In this case, left- and right-rotating waves are transmitted alternately in succession, and only one polarization direction is received. If, for example, left-rotating signals are received, the copolar signal components are obtained when transmitting the left-rotating wave, and subsequently the cross-polar signal components are obtained when transmitting the right-rotating wave. FIG. 11 illustrates the method according to the invention.

When using a plurality of transmitters, one obvious solution approach is to operate the transmitters chronologically in succession and to take the time-offset reception signals correspondingly into account in the signal evaluation. Because of the long transmission duration, however, a significant disadvantage arises. With a very long observation duration, a very good speed resolution is obtained, but high speeds can longer be determined uniquely.

The method according to the invention in this case provides a solution approach. A plurality of transmitters are operated simultaneously, and these are individually phase-encoded, the simultaneously operated transmitters always having the same polarization. For example, first all left-rotatingly polarized transmission signals are simultaneously transmitted while being phase-encoded, and subsequently, with a time offset, all right-rotatingly polarized transmission signals are transmitted while being phase-encoded. In general, the phase encoding may have different lengths. FIG. 12 illustrates the method according to the invention.

For the measurement of object heights at large range distances, there is a known method which relates to radar instruments that transmit linearly polarized signals. For measurement of objects located on the road, in this case a signal superposition takes place, which is caused by different propagation paths. In this case, the direct detection is superposed with a multipath propagation consisting of an additional reflection on the road surface and two so-called round trips. A round trip is intended to mean that the forward path and the return path differ from one another. Thus, in the first round-trip case, the forward path is the direct path and the return path involves the road reflection. In the second round-trip case, the forward path involves the road reflection and the return path is the direct path. By the superposition of different path propagations, a reception signal is obtained consisting of a superposition of back-reflected signals of the individual propagation paths. The reception signal therefore has an object-dependent characteristic profile as a function of the distance, this profile being determined depending on the object distance by sometimes constructive and sometimes destructive superposition of the different signals.

If objects which have a relative speed with respect to the sensor are followed as a function of the distance by means of a tracker, it is possible during the detection to determine the height of the object from at least two characteristic features, such as for example two minima of the reception signal. However, in the linear case, there is a significant disadvantage. Owing to the superposition of four different propagation paths, a characteristic curve is realized whose minima are locally very pronounced. Since, in the case of large distances, the back-reflected signal amplitude generally has a small distance from the noise level, the target can no longer be detected at a particular distance in the linear case because the reception signal lies below the noise level. FIG. 13 depicts the propagation paths that arise in the case of the known method, which relates to linearly polarized transmission signals.

In the method according to the invention of object heights, circularly or elliptically polarized signals are transmitted. In this case, either a left-rotatingly polarized or a right-rotatingly polarized signal is transmitted, but only the cross-polar reception signal is evaluated. The cross-polar signal, or the copolar signal, in this case always refer to the polarization direction in relation to the transmission signal. If the transmission signal is for example a left-rotating wave, the cross-polar reception signal is right-rotating and the copolar reception signal is left-rotating. For the sole evaluation of the cross-polar reception signal according to the method according to the invention, in contrast to the known approach with linearly polarized signals there are only two propagation paths, the back-reflected signals of which are superposed in the receiver. The propagation paths consist of direct detection and multipath propagation, which involves additional reflection on the road surface. In these two propagation paths, there are an odd number of reflections and the reception signal therefor appears in the cross-polar reception channel. The round trips occurring in the known method with linearly polarized signals are no longer present in the method according to the invention, since these signals occur in the copolar reception path because the number of reflections in the propagation path is even. The strongly pronounced minima occurring locally in the known method, which prevent object detection at particular distances, no longer occur in the method according to the invention, and the objects can be detected at all distances and the height can be carried out by evaluating typical features, for example two local minima at particular distances. FIG. 14 outlines the propagation paths in the method according to the invention for determining the object height. FIG. 15 shows typical characteristic curves as a function of distance in the known method, as well as in the method according to the invention. In this case, it is to be highlighted that the respective local minima fundamentally lie above the noise level. In contrast thereto, the local minima in the known method are often below the noise level and therefor difficult to identify. FIG. 16 shows the formula relationships for determining the object height.

A further application of the method according to the application and of the device according to the application consists in determining the friction coefficient of road surfaces, preferably by means of radar sensors, when for example additional reflections on the road surface are measured.

Particularly in the case of highly automated or autonomous driving, a predictive measurement of the friction coefficient of the road surface is required. The measurement results make it possible to establish speeds with which, for example, a curve may be safely navigated, without the risk arising that the vehicle will drift.

For the measurement of the friction coefficient, a circular or elliptically polarized radar sensor is required, which is mounted forwardly directed on the vehicle and two-dimensionally detects regions of the road or ground surface lying in front of the vehicle. This structure is shown in FIG. 17.

For the friction coefficient determination, the following procedure is required:

1. Calculating the local maxima for the co- and cross-polarized signals reflected back (left- and right-rotating signals)

2. Searching for an area which contains a particular road surface

3. Analyzing all local maxima of this region in respect of the following properties

-   -   amplitude magnitude ratio between co- and cross-polarized signal         components     -   phase difference between co- and cross-polarized signal         components     -   amplitude strength and polarization of the local maxima.

For illustration, it is in this case recommendable to plot this parameter in a graph. In FIG. 18, as an example, the results are represented for a road with a bitumen surface, which has a low roughness, and in FIG. 19 as an example the results for a graveled road with a naturally high roughness. The y axis respectively shows the amplitude magnitude ratio of the co- and cross-polar signal components, and the x axis shows the phase difference thereof. The cross-polar local maxima are represented as circles, and the copolar local maxima are represented as rectangles. The size of the circles and rectangles increases with the signal amplitude.

The friction coefficient of the road surfaces may then be determined by means of the following properties:

-   -   cluster formation, scattering     -   extent of the clusters     -   mean value of the phase difference positions of the clusters     -   mean value of the amplitude magnitude ratios of the clusters     -   standard deviation of the phase difference positions of the         clusters     -   standard deviation of the amplitude magnitude ratios of the         clusters     -   number of local maxima     -   signal amplitude of the local maxima.

According to FIG. 18 and FIG. 19, road surfaces with a low friction coefficient have the following properties in comparison with a road surface with a high friction coefficient:

-   -   small extent of the clusters     -   lower standard deviation of the phase differences and amplitude         magnitude ratios     -   lower signal amplitudes     -   smaller number of local maxima.

In the case of lower friction coefficients, these properties are caused by a similar manifestation of the back-scatter points and a similar orientation of the back-scatter points with respect to the sensor. In the case of very high friction coefficients, there are differently pronounced scatter points, which are oriented differently with respect to the sensor.

Furthermore, the phase position of the clusters makes it possible to analyze different surfaces more accurately, for example surfaces covered with snow, ice or leaves. In the case of layers of water on the road, the entirety of radar signals are reflected away and these may be identified by means of signal-free areas in the radar image, i.e. the absence of a radar signal, and dangerous aquaplaning situations may be identified.

Advantageous refinements are the subject-matter of the dependent claims. 

1. A method for object classification, the method comprising: a. providing an elliptically or circularly polarized transmission signal, which is transmitted onto the object to be classified b. generating a first radar image from the copolarly polarized reflection signal and generating a second radar image from the cross-polarized reflection signal; and c. comparing the first radar image with the second radar image.
 2. The method as claimed in claim 1, wherein the generation of the first and second radar images is carried out from at least one of left-circular signal components or right-circular signal components.
 3. The method as claimed in claim 1, wherein the generation of the first and second radar images is carried out from linearly horizontal and linearly vertical signal components.
 4. The method as claimed in claim 1, wherein the generation of the first and second radar images is carried out simultaneously, preferably by means of the transmission signal.
 5. The method as claimed in claim 1, wherein the comparison is carried out with the aid of the signal properties of the local maxima of the reflection signals.
 6. The method as claimed in claim 5, wherein the comparison is carried out with the aid of the signal properties of individual target regions of the object.
 7. The method as claimed in claim 5, wherein the signal properties comprise at least one of the following criteria: number of co-polar local maxima, number of cross-polar local maxima, magnitude ratio between co-polarization and cross-polarization, preferably average magnitude ratio thereof, maximum magnitude ratio between co-polarization and cross-polarization, minimum magnitude ratio between co-polarization and cross-polarization, phase ratio between co-polarization and cross-polarization, position of local maxima with a high or low magnitude ratio between co-polarization and cross-polarization, or speed evaluation.
 8. The method as claimed in claim 1, wherein similarity patterns are compiled for the object classification.
 9. The method as claimed in claim 1, wherein further object subclasses are provided, which are divided, at least in the case of a radar sensor transmitting the transmission signal, into distance from the radar sensor, angle orientation with respect to the radar sensor, object orientation with respect to the radar sensor, and relative speed with respect to the radar sensor.
 10. The method as claimed in claim 1, wherein the object classification relates to the position and orientation detection of the object, preferably an automobile.
 11. The method as claimed in claim 1, further comprising: determining a front objection; and in response to the case of frontal object determination, evaluating at least one of the following regions of an automobile: front region, front wheel wells, steering-wheel region, or side mirror.
 12. The method as claimed in claim 1, further comprising: transmitting with a time offset, by at least one transmission antenna, one of a left-circularly polarized wave and then a right-circularly polarized wave, or, a left-rotating elliptical wave and then a right-rotating elliptical wave; and evaluating only the left-rotating or the right-rotating signal component of the signals reflected back.
 13. The method as claimed in claim 1, further comprising: simultaneously transmitting, using a plurality of transmitters, identically polarized waves that are phase-encoded, wherein a transmission sequence is obtained that consists of simultaneous use of right-rotatingly polarized transmission signals and, with a time offset before or after, of simultaneous use of left-rotatingly polarized transmission signals.
 14. The method as claimed in claim 1, further comprising: determining, in addition to the object classification, the height of objects by detecting an object at different distances and evaluating the minima only of the reception signal component that is cross-polar in relation to the transmission signal.
 15. A device for establishing an object classification, the device comprising: a. means for providing an elliptically or circularly polarized transmission signal, which is transmitted onto the object to be classified; b. means for generating a first radar image from the copolarly polarized reflection signal and for generating a second radar image from the cross-polarized reflection signal; and c. means for comparing the first radar image with the second radar image arc provided.
 16. The device as claimed in claim 15, wherein means are provided for generating the first and second radar images from at least one of left-circular signal components or right-circular signal components.
 17. The device as claimed in claim 15, wherein means are provided for generating the first and second radar images from linearly horizontal and linearly vertical signal components.
 18. The device as claimed in claim 15, wherein the generation of the first and second radar images is carried out simultaneously, preferably by means of the transmission signal.
 19. The device as claimed in claim 15, wherein the comparison is carried out with the aid of the signal properties of the local maxima of the reflection signals.
 20. The device as claimed in claim 19, wherein the comparison is carried out with the aid of the signal properties of individual target regions of the object.
 21. The device as claimed in claim 15, wherein, with a time offset, a left-circularly polarized wave and then a right-circularly polarized wave, or, with a time offset, a left-rotating elliptical wave and then a right-rotating elliptical wave, are transmitted by at least one transmission antenna, and only the left-rotating or the right-rotating signal component of the signals reflected back is evaluated.
 22. The device as claimed in claim 15, wherein, identically polarized waves are transmitted simultaneously and phase-encoded using a plurality of transmitters, so that a transmission sequence is obtained which consists of simultaneous use of right-rotatingly polarized transmission signals and, with a time offset before or after, of simultaneous use of left-rotatingly polarized transmission signals.
 23. The device as claimed in claim 15, wherein, in addition to the object classification, the height of objects is determined by detecting an object at different distances and evaluating the minima only of the reception signal component which is cross-polar in relation to the transmission signal.
 24. The device as claimed in claim 15, wherein, in addition to the object classification, the friction coefficient of a road surface is determined by using a determined area of the road surface for analysis, and local maxima thereof are evaluated at least in respect of one of the following properties: cluster formation, scattering, extent of the clusters, mean value of the phase difference positions of the clusters, mean value of the amplitude magnitude ratios of the clusters, standard deviation of the phase difference positions of the clusters, standard deviation of the amplitude magnitude ratios of the clusters, number of local maxima, or signal amplitude of the local maxima. 